Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

pkgbase: Create a FreeBSD-elftoolchain package

It contain all the binaries and libs from the elftoolchain contrib
project except for libelf which is used everywhere.
All of those tools are never used by the average user.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38224

elftoolchain: Use ${SRCTOP} for the top of the FreeBSD tree

It's provided by sys.mk so there's no need to derive it from ${.CURDIR}.

Suggested by: ngie
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5998

Replace ${CURDIR}/../.. with ${CURDIR:H:H} in elftoolchain

This produces a nicer path in debug info and build logs.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

Build infrastructure for elftoolchain tools

Set WITH_ELFTOOLCHAIN_TOOLS in src.conf to use the elftoolchain version
of the following tools:

* addr2line
* elfcopy (strip / mcs)
* nm
* size
* strings

Reviewed by: bapt (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1224